题目

粗糙水平地面上有一个重为100N的物体，用20N的水平拉力使其在10s内匀速前进了10m，在此过程中（ ）A．重力做功的功率为100WB．支持力做功的功率为100WC．拉力做功的功率为200WD．拉力做功的功率为20W 答案：做功的两个必要因素：一是作用在物体上的力，二是在力的方向上移动的距离，先判断重力、支持力和拉力是否做功，再利用功的公式求做功大小． 【解析】 重力的方向是竖直向下的，物体水平移动，在重力的方向没移动距离，故重力不做功，重力做功功率为0，故A错； 支持力的方向是竖直向上的，根据对话内容，从方框内的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。 A: Hi! Lu Ming. I’m really glad to hear that you’ve done so well in the college entrance examination. I          1          . B: Thank you! A: With such good results, you will be able to get into Beijing University, I’m sure.            2       B: I will study Medicine.        3       A: It’s a good major(专业). Next year I will finish middle school.      4       B: So from now on , you must make full use of your time to get well prepared（准备） for next year’s college entrance examination. A: I’ll try my best to do as well as you have done. B:        5          A: Thanks a lot! A. Congratulations. B. What’s your job? C. I’m sure you will! D. You must do better than I’ve done. E. What major will you choose? F. And I want very much to enter Tsinghua University. G. It has been my dream to become a doctor all these years.